Current measuring apparatus employing magnetic switch

ABSTRACT

Current measuring apparatus for determining the strength of the current in an electric conductor. The apparatus comprises an actuating coil, a magnetic switch positioned within the composite magnetic field produced by the currents flowing through the conductor and the actuating coil, generating means coupled to the actuating coil for providing an increasing exciting current or an increasing step-wise voltage thereto, and measuring apparatus coupled to the magnetic switch for determining the instant at which the switch changes from one state to the other under the influence of the composite magnetic field. The current through the actuating coil at the instant that the switch changes states corresponds to the magnitude of the current through the electric conductor. The apparatus may be used in conjunction with the operation of electrolytic cells having large numbers of anodes wherein it is useful to know the current flowing through each anode.

I v v United States Patent 1 [111 3,801,908

Van Andel Apr. 2, 1974 CURRENT MEASURING APPARATUS PrimaryExaminer-Alfred E. Smith EMPLOYING MAGNETIC SWITCH AssistantExaminer-Emest F. Karlsen [75] Inventor: Eleonoor van Andel, TWkkelo,Attorney, Agent, or Firm-Stevens, Davis, Miller &

Netherlands Moshe [73] Assignee: Akzo N.V., Arnhem, Netherlands [57]ABSTRACT [22] Filed: Mar. 9, 1972 Current measuring apparatus fordetermining the App]. No.: 233,189

strength of the current in an electric conductor. The apparatuscomprises an actuating coil, a magnetic switch positioned within thecomposite magnetic field produced by the currents flowing through theconductor and the actuating coil, generating means coupled to theactuating coil for providing an increasing exciting current or anincreasing step-wise voltage thereto, and measuring apparatus coupled tothe magnetic switch for determining the instant at which the switchchanges from one state to the other under the influence.of the compositemagnetic field. The current through the actuating coil at the instantthat the switch changes states corresponds to the magnitude of thecurrent through the electric conductor. The apparatus may be used inconjunction with the operation of electrolytic cells having largenumbers of anodes wherein it is useful to know the current flowingthrough each anode.

9 Claims,'4 Drawing Figures H PATENIEMPB -2 Ian SHEET 1 OF 2 PATENIEDAPR2 I974 I SHEET 2 BF 2 FIG.4

TIME

STEP-WISE POWER SOURCE VOLTAGE wl l'k- MEASURING APPARATUS BACKGROUND OFTHE INVENTION The invention relates to a current-measuring apparatus fordetermining the strength of current in one or more electric conductorsby means of a magnetic switch positioned near said conductors.

A known current-measuring apparatus of the abovementioned type comprisesa number of reed switches which are positioned at various distances fromthe conductor and whose contacts are closed under the influence of themagnetic field prevailing around the conductor. According to thedistance between the conductor and the reed switch, each of the reedswitches responds to a particular current strength. The moment thecontacts of a reed switch are closed, a corresponding lamp will lightup. The higher the current strength, the larger the number of lamps thatwill light up, and conversely. The known current-measuring apparatuspermits only a rough measurement of the current strength I. If, forinstance, it comprises three reed switches which respond to somestrength of current in the conductor of l l and l,,-amp., then it isonly possible to indicate in which of the four following ranges thecurrent strength 1 lies:

2. I I I It will be clear that this type of measurement is veryinaccurate. Its inaccuracy can only be reduced by providing a largenumber of reed switches placed relative to the conductor at radialdistances which must differ only little. In that case, however, a largenumber of reed switches are required which presents difficultiesparticularly if a large number of conductors are present. This is thecase for example in the electrolysis of NaCl. In an electrolytic plantthere are present a large number of electrolytic cells, each consistingof a longshaped trough having a somewhat inclining bottom over whichthere is a flow of mercury. The mercury is covered with brine. The cellis closed at its top by a lid through which there project a number ofanodes which extend downwards to just above the level of the mercury.Upon the passage of current from the anodes to the mercury cathode, theNaCl decomposes into sodium and chlorine which are liberated at thecathode and the anode, respectively. The sodium unites with part of themercury to form sodium amalgam, which in a caustic soda cell connnectedto the electrolytic cell reacts with water to form sodium hydroxide andhydrogen. The mercury thus recovered may be fed back to the electrolyticcell.

In order -to ensure proper operation, the anodes should be correctlyspaced in relation to the mercury cathode. If the distance is too small,there is the risk of a short circuit taking place, which may bedisastrous, especially if use is made of metal anodes. Moreover, a lossof production may be caused by the chlorine reuniting with the sodiumfrom the sodium amalgam to form NaCl. On the other hand, too large adistance leads to an increased voltage drop between anode and cathode,which with the high current strengths that are used leads to aconsiderable increase in electric energy consumption.

Monitoring the electrolysis process by frequent measurement of thedistance between the anodes and the cathode is difficult to realizebecause the electrolytic cell is entirely closed off. Besides, theanodes and the cathode are not continuously equidistant since both themercury surface and the opposed surface of the underside of the anodesare usually uneven. For process monitoring, it is therefore preferred tomeasur the cell current, and more particularly to measure the currentthrough each anode separately, because the distribution of cell currentmay be highly non-uniform among the anodes.

It is known that the anode current may be determined from the drop ofpotential measured between two points on the anode current conductor. Asthis SUMMARY OF THE INVENTION The invention comprises current measuringapparatus including an actuating coil cooperating witha magnetic switch,an electric current source which is connected to the actuating coil andserves to supply an exciting current varying with time, and a measuringapparatus for determining the strength of the current in the actuatingcoil at the moment the switch changes its switching state under theinfluence of the combined magnetic action of the current in theconductor and the current in the actuating coil.

A magnetic switch, as defined herein, is an element which under theinfluence of a magnetic field has two different electric states.

In this current-measuring apparatus, an external magnetic field isproduced by the current in the conductor and forms a composite fieldtogether with the internal magnetic field produced by the actuatingcoil. When the strength of this composite field has reached a particularthreshold value, the magnetic switch is brought into other state. Asthis threshold value is reproducible, the strength of the current in theactuating coil at the moment the switching state is changed is also ameasure of the strength of the external magnetic field and, hence, ofthe strength of the current in the conductor.

A conceivable magnetic switch may comprise a ring core placed on theconductor and provided with two windings, the core consisting of amagnetic material with a rectangular hysteresis loop. One of thewindings serves as an actuating coil and the other as a detection coil.When there is no flow of current through the actuating coil, the corewill be saturated. Upon the passage through the actuating coil ofcurrent of increasing strength in a direction such that the fieldstrength in the core decreases, the magnetic flux in the core will at agiven moment reverse in direction. This reversal induces a voltage pulsein the detection coil. The current strength prevailing in the excitingcoil at the moment the voltage pulse is induced may serve as a measureof the strength of the current in the conductor.

However, it is preferred to use a current-measuring device which ischaracterized in that the magnetic switch is formed by a reed relayprovided with an actuating coil.

The reed relay consists of a pair of magnetic reeds sealed into a lengthof glass tubing filled with an inert gas. By applying a magnetic fieldparallel to the tubing, the reeds will be moved into contact with eachother and close the current circuit connected to the reed relay. Thereed relay is an inexpensive and reliable device. It may be sopositioned relative to the conductor that it does not close until thestrength of the magnetic field produced around the conductor has beenincreased to a given value by the field of the actuating coil.

It is preferred to place the reed relay in such a position, relative tothe direction of the current in the conductor, that it remains closedwhile under the influence of the field of the current-carrying conductoronly. This is because the opening of the reed relay can be reproducedbetter than the closing.

In this connection, it is recommended that the current source suppliesan increasing current. The current source is so connected that thedirection of the magnetic field produced by the current-carryingactuating coil is opposite to that of the field produced by thecurrent-carrying conductor and the exciting current is increased untilthe magnetic switch changes its state to open the reed relay. At thatmoment, the magnetic field of the actuating coil has compensated thefield of the conductor.

The strength of the current in the actuating coil at which the magneticswitch passes from the one switching state into the other, may be variedby applying pre-magnetization with field. This may be the field of apermanent magnet or electromagnet or that of a separately fed,additional actuating coil.

In principle, the exciting current may be increased as some function oftime, provided that at any moment the current strength is accuratelyknown. If the currentmeasuring apparatus serves to determine thestrength of the currrent in only one or a few conductors, then asuitable solution would be to provide a supply source which furnishes acurrent that increases linearly with time. The measuring sensitivitywill then be independent of the strength of the exciting current and themeasurement of the current strength can be reduced to a simple timemeasurement with the aid of a counter. This counter is started at thesame time as the current supply source and stopped at the moment themagnetic switch changes its state.

The invention also relates to an electric distribution system consistingof a number of current conductors for feeding electrical equipment, moreparticularly an electrolytic cell, which distribution system is providedwith a current-measuring apparatus of the aforedesc'ribed type. Thisdistribution system is character ized in that at least part of thecurrent conductors are provided with a magnetic switch with an actuatingcoil cooperating therewith, the actuating coils are connected to acommon electric source, and the magnetic switches are connected to acommon measuring apparatus.

the aid of a third magnetic where (AE) i is the voltage change at theith step.

The common current source may be successively connected to the variousactuating coils, after which at each actuating coil the current strengthis determined at which the corresponding magnetic switch is opened. Asimpler arrangement is to connect the actuating coils in series with thecommon current source, all coils then carrying the same current.

However, it is preferred to connect the actuating coils in parallel sothat the same voltage is applied to all coils. Surprisingly, adetermination of the voltage at which the magnetic switches change theirswitching state results in a more accurate measurement of the strengthof the current in the conductor than obtained by a determination of theexciting current. This is apparently due to some variation in thenumbers of windings of the various actuating coils. As the compensationof the field around the conductor not only depends on the excitingcurrent but also on the number of windings of the respective actuatingcoil, a measuring error may occur if this number differs from theexpected number. A preferred embodiment of the electric distributionsystem with the actuating coils connected in parallel is characterizedin that the current supply source furnishes a voltage which increasesstepwise. This embodiment is particularly suitable in cases where thedistribution system comprises a large number of conductors for which thecurrent strengths have to be determined. In such a case, thesimultaneous measurement of all current strengths may be objectionable.It would be better then for the switching conditions of the magneticswitches to be read in rapid succession. In the case of a stepwiseincrease in the feed voltage a certain time is available between twosuccessive voltage steps for deciding which switches have changed theirswitching state as a result of the preceding step. This may be doneseparately for each switch; but the positions of the relays also may beread groupwise i.e., in such a way that of each group of relays theinformation about the switching state becomes simultaneously availablefor all relays of the group.

If the voltage after the ith voltage step is E, and after the subsequent(i l the step is E, l and a magnetic switch changes its switching stateafter the ith step, the corresponding voltage E of its actuating coil isdetermined by the relationship Therefore, the measuring apparatus hasfor its task to decide after each step which switch (or switches) haschanged its switching state. The current strength of the correspondingconductor is then derived from (1).

The accuracy with which E is determined may be influenced by the stepmagnitudeAE. The voltage steps need not all be of the same magnitude. Insome cases it is preferred that the magnitude of the voltage stepsshould be variable. This applies particularly in those cases where thecurrent strengths in the conductors are on the whole within a relativelylimited measuring range. In that case it is recommended that within thisrange smaller voltage steps are used than outside this range.

The invention also relates to an electric distribution system forfeeding one or more electrolytic cells which are each provided with atleast two electrodes. It is characterized in that on at least onecurrent conductor connected to an electrode there is provided switch.

Reed relays are already known to be used on electrolytic cells for thepurpose of preventing a short circuit. In that case a reed relay isprovided on the busbar for carrying the current to an anode sub-groupconsisting of, for instance, five anodes. The reed relay is sopositioned on the busbar that in normal operation it is open, but assoon as the total current for the anode subgroup exceeds a certain valueit is closed. The respective anode sub-group is then lifted and removedfrom the cathode. In order that afterwards the anodes may again beplaced at the correct distance from the cathode, the current in thebusbar is measured by determining the drop of potential between twopoints of the busbar. Therefore, in this known apparatus the reed relaysdo not serve to measure the anode current but merely to detect animminent short-circuit.

Further, the invention relates to an electric distribution system forfeeding one or more electrolytic cells which are each provided with anumber of anodes and one common cathode, more particularly for feedingan electrolytic cell with a mercury cathode. This system is a magneticcharacterized in that on each current conductor connected to an anodethere is provided a magnetic switch.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 isa cross-sectional view of areed relay;

FIG. 2 is a graph showing the operation of the reed DESCRIPTION OF THEPREFERRED EMBODIMENTS In FIG. 1, the numeral 1 refers to a glass tube ofa reed relay filled with inert gas. Into the tube 1, which has alongitudinal axis x-x,.are sealed two magnetic reeds 2, 3 whoseprojecting ends 4 and 5 serve as electrical connections. Around the tube1 there is wound an actuating coil 6 to which current may be fed viaconnections 7, 8. When there is a flow of current through the coil aninternal magnetic field will be produced which magnetizes the reeds 2,3. When a certain cur rent strength is exceeded, the reed ends 9, '10are moved into contact with each other and the reed relay is closed.When subsequently the exciting current decreases, the contact is againopened.

If the reed relay is provided on or near a currentcarrying conductor,i.e., with its longitudinal axis x-x transverse to the direction of flowof the current, then the reed relay also may be closed under theinfluence of the field around the conductor. If, in such a situation,current of increasing strength is fed to the actuating coil, then thereed relay will at a given moment open again when the direction of themagnetic field is opposite to that of the conductor.

FIG. 2 shows the behavior of the reed relay under the combined influenceof the field produced by the current-carrying actuating coil and anoppositely directed external field of different origin. 0n the abscissais plotted the current strength i in the actuating coil, on the ordinatethe magnetic field strength H of the external field. Let the initialexciting current i 0, the

strength of the external field H A at which the reed relay is closedbeing indicated by the point A. When the exciting current increases, thepoint (H i) will move along a straight line parallel to the abscissa. Ata current strength i the reed relay (point B) opens. Then the field ofthe actuating coil compensates the external field. Upon a furtherincrease in current, the reed relay remains open until the currentstrength has reached the value i (point C). The field of the actuatingcoil will then sufficiently dominate the external field to cause thereed relay to close again.

So three areas may be distinguished, which are shown differently hatchedin FIG. 2. In the areas I and III the reed relay is closed; in the areaII it is open. The dividing line between the areas I and II isindicative of a unique relation between the strength H of the externalmagnetic field and the current strength i in the actuating coil requiredfor the compensation of this field. Thus, from the compensating currentstrength I in the actuating coil the current strength in the conductorcan be derived. The value I may be determined with the aid of ameasuring apparatus.

The figure does not show the influenceof contact hysteresis. But thelatter does not affect the measurement provided that the operation graphof FIG. 2 is always followed in the same direction.

Around the conductor whose current is to be measured, there may beprovided a U-shaped iron circuit between the ends of which the reedrelay is placed. The lines of force in the magnetic field of thecurrentcarrying conductor run via this iron circuit and the reed relayplaced between the end thereof. This construction may be of importancefor measuring direct current of relatively low strength. For relativelyhigh current strengths there is no need for this provision since a reedrelay closes at a distance of about 1 meter from a conductor carrying acurrent of 1,000 amps. Moreover, without the use of an iron circuit abetter linear relation is obtained between exciting current and currentto be measured.

In order to avoid interference from disturbing fields, various reedrelays may be arranged symmetrically around the conductor. The meanvalue of the exciting currents required to open the reed relays may betaken as a measure of the current strength to bedetermined.

An embodiment in which the above-described measuring principle isapplied to an electrolytic cell is partly shown in FIG. 3.

In this figure the numeral 11 refers to a part of a lid of a cell forthe electrolysis of NaCl. In the cell are a number of graphite anodeswhose undersides are near a flowing mercury cathode. On the mercury isbrine. Direct current is fed to the anodes via an electric distributionsystem, which consists of current conductors that are connected to anodebars, one of which is shown in FIG. 3, which are attached to the anodes.This anode bar consists of a graphite sleeve 12 fitting on a copper pin13. A terminal 14 is attached to the pin 13 by means of a bolt 15.Attached to the terminal 14 are a number of strip-shaped conductors l6,l7 and 18,

which by a suitable connector (not shown) are connected to a busbar. Theconstruction comprising the conductors 16, 17 amd 18, the terminal 14,and the copper pin 13 with graphite sleeve 12 can be bodily moved up ordown for the adjustment of anodecathode distance. A seal 19 serves toprevent chlorine from escaping viathe anode passage through the lid 1 l.

Underneath the conductors l6, l7 and 18, a reed relay is positionedagainst the sleeve at right angles thereto. As described with referenceto FIG. 1, this reed relay comprises an actuating coil 6 with a glasstube 1 containing two contact reeds. One of the contact reed connectionsis visible in the drawing and referred to by 5. The connections of thecoil 6 are indicated by 7 and 8.

On each anode bar of the electrolytic cell and also on each anode bar ofany other electrolytic cell, if present, there is provided a reed relay.All of these reed relays are connected in parallel with a commonelectric current source which is present in a control room. The supplysource furnishes a step-wise increasing voltage to the actuating coils.The magnitude of the steps is not constant. As soon as the excitingvoltage has reached a value which falls within a range which correspondsto the operating range of the anodes, the magnitude of the step isreduced. By operating range of the anodes is to be understood here, therange of the current strength within which lie most of the anode currentstrengths. By increasing the voltage applied to the actuating coils thereed relays will be opened. In order to determine the anode currentstrengths a measuring apparatus, which is also present in the controlroom, need only determine at what exciting voltages the various reedrelays were opened. The measuring apparatus may serve for all reedrelays. It may read the switching positions of the reed relays one byone or groupwise. In the latter case the reed relays are combined intogroups, the measuring apparatus simultaneously reading the switchingpositions of the reed relays belonging to a particular group.

For only four reed relays 2l24 the situation is schematically shown inthe circuit diagram of FIG. 4. These relays comprise the magnetic reedcontacts 28 and the exciting coils 29-32. The coils 29-32 are connectedin parallel to a common electric current source 33 in a control room 34.The voltage supplied by the current source 33 increases stepwise. Alsopresent in the control room is a measuring apparatus 35 to which thereed contacts 25-28 are connected by means of wires 36-40. Wire 36 isconnected to all four relay contacts and is at ground potential. As soonas the voltage across one of the relay coils, say 30, has increased tosuch a value that the corresponding relay contact 26 opens, connectingwire 38 is no longer at ground potential, whereby the opening of relaycontact 26 is indicated on indicators l. Thereupon the output voltage ofcurrent source 33 is'determined and serves as a measure of the currentstrength in the anode to which reed relay 22 is attached.

It is of importance that the information which is to be supplied fromthe anodes to the control room is only of a binary nature": contactclosed or open. In this way the transfer of signals may be considerablysimplified. If the switching state of the reed relays between twosuccessive voltage steps is sensed in rapid succession, then for nelectrodes the use of a cable consisting of 2 log n wires will suffice.For 6,400 anodes, for example, this implies that the cable running fromthe reed relays may consist of 26 wires.

If desired, the current strength may in the abovedescribed way bemeasured in the main conductors from which the anode current conductorsbranch off.

Of course, the entire measurement and the processing of the measuringdata may be automated. To this end the increasing of the excitingvotage, the reading of the reed relays, and the registration of theexciting voltages at which they are opened may be controlled from acentral point.

The entire measuring and registration procedure may be carried out incycles of fixed duration or automatically be restarted as soon'as thepreceding cycle is completed. Moreover, it is, of course, possible forthe adjustment of the distance of the anodes to the cathode on the basisof the measurements of the strength of the anode current to be effectedmanually or automatically. The above procedures may with advantage becarried out under the control of a digital process computer,particularly if they are to be carried out in an alectrolytic plant witha large number of cells.

Although in the foregoing description mention is made of a commonelectric current supply source and a common measuring apparatus, thisdoes not mean to imply that all reed relays must cooperatesimultaneously with the current supply source and the measuringapparatus. It is also possible for the supply source and the measuringapparatus to be connected cell by cell to the reed relays providedthereon.

The above-described measurement of current strength has the greatadvantage that the actual current pickup, i.e., the reed relay, is avery reliable and inexpensive element. It is therefore possible toprovide each anode with a separate pickup without this leading to aprohibitively costly apparatus. Thus at each cell the anode currents andconsequently also the distribution of current among the various anodesmay be observed; moreover, protection from short-circuiting may beobtained, which is of particular importance if use is made of costly.titanium anodes. Besides, the binary character of the pickupconsiderably simplifies data processing. It is of further advantage thatthe measuring circuit is electrically isolated from the electrolyticcell.

What is claimed is:

1. Current measuring apparatus for determining the strength of thecurrent in at least one electric conductor comprising a. at least oneactuating coil for producing a magnetic field corresponding to anexciting current therethrough,

b. at least one magnetic switch having a pair of switching statespositioned within the composite magnetic field produced by the currentsflowing through said conductor and said actuating coil respectively,

c. generating means coupled to said actuating coil for providing anexciting current thereto which varies with time, and

d. measuring apparatus coupled to said magnetic switch for determiningthe instant at which said switch changes from one state to the otherunder the influence of the combined magnetic action of the currents insaid conductor and said actuating coil, the current in said actuatingcoil at said instant corresponding to the current through said electricconductor.

2. Current-measuring apparatus according to claim 1 wherein saidactuating coil and said magnetic switch comprise a reed relay.

3. Current-measuring apparatus according to claim l whereinsaidgenerating means provides an increasing actuating current.

4. An electric distribution system having a plurality of currentconductors for feeding at least one electrolytic cell comprising a. aplurality of actuating coils for producing a magnetic fieldcorresponding to an exciting current therethrough,

b. a plurality of magnetic switches, each having a pair of switchingstates and each positioned within the magnetic field produced by thecurrent flowing through a corresponding conductor and the magnetic fieldproduced by a corresponding actuator coil,

c. generating means coupled to said actuating coils for providingexciting currents therein which vary with time, and

d. measuring apparatus coupled to each of said magnetic switches fordetermining the instant at which said switch changes from one state tothe other under the influence of the combined magnetic action of thecurrents in said corresponding conductor and corresponding actuatorcoil, the current through said actuating coil at said instantcorresponding to the current through said electric conductor. v

5. An electric distribution system according to claim 4 wherein saidactuating coils are connected in parallel.

6. An electric distribution system according to claim 5 wherein saidgenerating means provides a voltage which increases stepwise. I

7. An electric distribution system according to claim 6 wherein themagnitude of said voltage steps is variable.

8. An electric distribution system according to claim 2 wherein saidreed relay is mounted adjacent said conductor with its longitudinal axistransverse to the direction of current flow in said conductor.

9. The method of measuring current in a conductor comprising the stepsof a. placing a switch responsive to a magnetic field and having a pairof switching states within a first magnetic field produced by thecurrent flowing through said conductor,

b. generating a second magnetic field which surrounds said switch, saidfirst and second magnetic fields forming a composite magnetic field,

0. increasing the strength of said second magnetic field until saidswitch changes from one state to the other under the influence of saidcomposite magnetic field, and

d. measuring the strength of said second magnetic field at the instantsaid switch changes from one state to the other, the strength of saidsecond magnetic field at this instant corresponding to the magnitude ofthe current in said conductor.

1. Current measuring apparatus for determining the strength of thecurrent in at least one electric conductor comprising a. at least oneactuating coil for producing a magnetic field corresponding to anexciting current therethrough, b. at least one magnetic switch having apair of switching states positioned within the composite magnetic fieldproduced by the currents flowing through said conductor and saidactuating coil respectively, c. generating means coupled To saidactuating coil for providing an exciting current thereto which varieswith time, and d. measuring apparatus coupled to said magnetic switchfor determining the instant at which said switch changes from one stateto the other under the influence of the combined magnetic action of thecurrents in said conductor and said actuating coil, the current in saidactuating coil at said instant corresponding to the current through saidelectric conductor.
 2. Current-measuring apparatus according to claim 1wherein said actuating coil and said magnetic switch comprise a reedrelay.
 3. Current-measuring apparatus according to claim 1 wherein saidgenerating means provides an increasing actuating current.
 4. Anelectric distribution system having a plurality of current conductorsfor feeding at least one electrolytic cell comprising a. a plurality ofactuating coils for producing a magnetic field corresponding to anexciting current therethrough, b. a plurality of magnetic switches, eachhaving a pair of switching states and each positioned within themagnetic field produced by the current flowing through a correspondingconductor and the magnetic field produced by a corresponding actuatorcoil, c. generating means coupled to said actuating coils for providingexciting currents therein which vary with time, and d. measuringapparatus coupled to each of said magnetic switches for determining theinstant at which said switch changes from one state to the other underthe influence of the combined magnetic action of the currents in saidcorresponding conductor and corresponding actuator coil, the currentthrough said actuating coil at said instant corresponding to the currentthrough said electric conductor.
 5. An electric distribution systemaccording to claim 4 wherein said actuating coils are connected inparallel.
 6. An electric distribution system according to claim 5wherein said generating means provides a voltage which increasesstepwise.
 7. An electric distribution system according to claim 6wherein the magnitude of said voltage steps is variable.
 8. An electricdistribution system according to claim 2 wherein said reed relay ismounted adjacent said conductor with its longitudinal axis transverse tothe direction of current flow in said conductor.
 9. The method ofmeasuring current in a conductor comprising the steps of a. placing aswitch responsive to a magnetic field and having a pair of switchingstates within a first magnetic field produced by the current flowingthrough said conductor, b. generating a second magnetic field whichsurrounds said switch, said first and second magnetic fields forming acomposite magnetic field, c. increasing the strength of said secondmagnetic field until said switch changes from one state to the otherunder the influence of said composite magnetic field, and d. measuringthe strength of said second magnetic field at the instant said switchchanges from one state to the other, the strength of said secondmagnetic field at this instant corresponding to the magnitude of thecurrent in said conductor.